The present invention relates to control circuits for television receivers. More particularly, the invention relates to an improved vertical centering control circuit for a television receiver.
In the vertical deflection circuit for a television receiver, a sawtooth sweep signal is applied to the deflection drive amplifier by means of a time constant circuit which has a long time constant in order to linearize the ramp portions of this 60HZ sweep signal. The long time constant, however, produces deleterious effects on the deflection circuit. First of all, the long time constant affects the response time of the vertical deflection drive amplifier to changes in the centering control voltage. Thus, when the television receiver is being assembled and the picture on the screen is being vertically centered, an adjustment of the centering control results in a substantial time delay before the picture position fully responds to the adjustment. Of course, if the initial adjustment did not accomplish the desired positioning of the picture, additional adjustments will have to be made, each adjustment occasioning a delay before its full effect is realized. These delays are, of course, costly in a high volume production line.
Secondly, variation in the voltage level of the power supply for the vertical deflection amplifier results in momentary picture position shift. This is due to the fact that shift in the voltage level is promptly tracked by the vertical deflection drive amplifier, but the corresponding change in the centering voltage level is not promptly responded to due to the delay occasioned by the aforementioned time constant circuit.
The third problem resulting from the long time constant circuit is the expense associated with the large-valued components, especially the capacitor incorporated therein.
It is contemplated as an object of the present invention to avoid the aforementioned problems resulting from the time constant circuit of the prior art. This is achieved by replacing the resistor in the time constant circuit with a diode or other device having a diode characteristic, namely, a device having a high impedance under one bias condition and a low impedance under a second bias condition. In one embodiment of the present invention, the time constant circuit including the diode is utilized to couple not only the sweep signal to the deflection drive amplifier but also to couple the vertical centering control to the amplifier. The positioning of the diode in the circuit is such that both the high impedance and low impedance states of the device is employed so that the amplifier will quickly respond to DC voltage changes while at the same time a long time constant is provided for linearity of the sweep signal. In a second embodiment, the diode and the centering control interact through the two input terminals of a differential amplifier input stage for the deflection drive amplifier.
Accordingly, it is an object of the present invention to provide a dual purpose time constant circuit for the vertical deflection drive amplifier of a television receiver which enables the amplifier to promptly respond to DC voltage level change and also to linearize the sweep signal applied to the amplifier.
Another object of the present invention is to provide a time constant circuit employing a diode to provide a long time constant to linearize the sawtooth sweep signal applied to the vertical deflection drive amplifier and also to isolate this amplifier from the effects of the long time constant during DC supply voltage level changes.